Avalanche photo-diode (APD) is a solid-state photo-sensor with internal gain.
When light signal is applied to APD, it generates current (I) that equals a product of a multiplication of the power (P) of light that impinges on the APD, the photo-sensitivity (S) of the APD and the gain (M) of the APD. The Gain (M) is also referred to as an internal gain of the APD.
The power (P) may be measured in Watts [W], the photo-sensitivity (S) may be measured in Ampere per Watts [A/W] and the gain (M) may be measured in Ampere per Ampere [A/A].
The Value of the gain depends on a value of a voltage applied to the APD (hereinafter APD voltage or VAPD) and the APD junction temperature. This dependence is especially strong for high values of the gain. For simplicity of explanation it is assumed that the APD temperature is the temperature of the APD junction on which light impinges. The APD junction temperature is referred to as APD temperature. It is known in the art that the ability to use an APD in high gain applications directly depends on the stability of the APD voltage and voltage noise as well as on the stability of the APD temperature. For example, highly sensitive optical systems may aim for using APD at a gain of 500 and at APD current (IAPD) of 300 microAmpere. Under such requirements, APD junction temperature variation of about 0.05 Celcius may cause an APD current variation (which, for some application, represent an error) of 1/256=0.004=0.4%. A 0.4% error is high enough in order to interfer the correct work of highly sensitive optical systems. Known APD-based systems support APD temperature variations of about 1.0 Celcius, which increases error level above and beyond the requirements of certain highly sensitive optical systems.
The following publications relate to APD, operation of APD at high gain and APD temperature: International Publication Nos. WO2007/030734; WO2003/069379. U.S. Pat. Nos. 4,599,527; 5,696,657; Perez Garcia M. A. et al, “Low-cost Temperature Stabilization in APD Photo Sensors by means a High Frequency Switching DC/T Converter”, IEEE Instrumentation and Measurement Technology conference Anchorage, Ak., USA, 2 1-23 May 2002; Tamer F. Refaat “Temperature Control of Avalanche Photodiode Using Thermoelectric Cooler”, NASA/TM-1999-209689, October 1999; “Voxtelopto NIR Photodiodes Photoreceivers Catalog” by Voxtel™, Inc. 2011; “Spec Sheet: Advanced Photonix OEM Modules—Cooled”, http://proddownloads.vertmarkets.com/download/facdabea/facdabea-b890-44cf- -8e01-2354c1f4c0cf/original/oemcooled.pdf; “Microelectronucs Receiver TIA with TEC and HV Regulator, 200 μm InGaAs APD (Avalanche Photodiode) 264-339746-001” by CMC Electronics™.
It is noted that APD temperature is influenced by the APD average current (static current) and by fast APD current changes (dynamic current). Furthermore, in order to prevent APD damage the level of the average APD current should be limited. For example, the average APD current may be limited to a level of few tens of micro-Amperes till few hundreds micro-Amperes (for example, 500 micro Ampere). An APD current dynamic range in practice may be five decades, i.e. 100,000, or even more, and APD current's frequency range may be in the range of few tens of GHz.
The electrical power dissipated on an APD (P) equals IAPD*VAPD. This electrical power is directly converted to heat. If the APD is maintained at a fixed gain then the value of the APD voltage is constant. Therefore, heat dissipated on the APD may change, for example in the range of 100,000 times.
It has been found that the APD temperature changes over time and this induces changes in the gain of the APD. Thus, static or/and dynamic non-linearity of APD response are experienced and this is undesired for certain applications.
Typically, The APD voltage may be set to values between 0V and 500V, depending on required APD gain (the higher limit may be between 5V and few thousand of volts 3,000V for different technologies of APD). Together with wide APD current dynamic range (static and dynamic) it sets significant challenge to designer of bias voltage supply system.
The APD gain may be between 1 A/A and few thousands A/A. For APD gain in the range of few hundreds and for an allowed error of not more than 1/256=0.4%, the required stability of the APD voltage is in range of few tens of mV peak-to-peak (voltage domain) and the required stability of the APD temperature is in range of few tens of milli-degrees (temperature domain). For gains of one thousand and more above requirements are even tighter.
FIG. 1 is a schematic diagram of a prior art device 201 that includes: (a) Controller 19. (b) Direct current to direct current (DC-DC) converter 11 that serves as a high voltage supply module for providing the APD voltage (VAPD 102). DC-DC converter 11 is controlled by a control signal APD HV set 101 that is supplied by controller 19. (c) APD 13. (d) First capacitor C1 12 that filters the voltage supplied to APD 13. (e) Trans-impedance amplifier (TIA) 14 that includes amplifier U1 11 and a feedback resistor R11 15. TIA 14 is arranged to output via output port 16 an output voltage VOUT 104, wherein VOUT=IAPD*R11. FIG. 1 also shows load resistor such as Rload 17 that is connected to output port 16.
In voltage domain device 201 may suffer from the following problems: DC-DC converter 11 usually has a slow load regulation response (in the range of DC-DC switching frequency, which is about 100 KHz); The time response of the DC-DC converter's output current limiting circuit is slow (in the range of DC-DC switching frequency, which is about 100 KHz); DC-DC converter 11 usually has a high output ripple and noise.
In the temperature domain—the prior art device 201 may suffer from the following problems: There are no special means for APD junction temperature stabilization; therefore, APD 13 may be used with relatively low gains (up to few tens) without to sacrifice APD gain linearity.
FIG. 2 illustrates prior art device 202. Device 202 is connected to a load that is represented by Rload 17.
Prior art device 202 includes: (a) Controller 19. (b) DC-DC converter 11. DC-DC converter 11 is controlled by (i) control signal APD HV set 101 that is supplied by controller 19 and by (ii) an offset signal 105 provided from temperature feedback module 22. (c) APD 13. (d) First capacitor C1 12 that filters the voltage supplied to APD 13. (e) TIA 14. (f) Temperature sensor (TS) 30 for sensing the temperature of APD 13. (g) Temperature feedback module 22 that receives temperature readings from TS 30 and outputs temperature offset signal 105 for compensating for changes in the temperature of the APD 13. This circuit may be included in controller 19 or be separated from the controller 19.
Prior art device 202 allows at least a limited amount of compensation for temperature changes. In voltage domain and for certain applications, this configuration may show the following disadvantages: DC-DC converter 11 usually has slow load regulation response (in the range of tens KHz). The time response of DC-DC converter's output current limiting circuit is slow (in the range of tens KHz). The DC-DC converter 11 usually has high output ripple and noise.
In temperature domain and for certain applications this configuration has following disadvantages: The function, realized by temperature feedback module 22 is complicated (APD gain M depends on both HV and APD temperature), and may be realized properly only in microcontroller with multi-dimensional look-up table (LUT). The time response of the temperature feedback module 22 is slow (in the range of tens KHz).
FIG. 3 illustrates prior art device 203. Device 203 is connected to a load that is represented by Rload 17.
Device 203 includes: (a) Controller 19. (b) DC-DC converter 11. DC-DC converter 11 is controlled by control signal APD HV set 101 that is supplied by controller 19. (c) APD 13. (d) First capacitor C1 12 that filters the voltage supplied to the APD 13. (e) TIA 14. (f) Thermoelectric cooler (TEC) 40 that includes cold plate 41, hot plate 42 and solid state devices 43. Solid state devices 43 transfer heat from cold plate 41 to hot plate 42 under the control a TEC controller 44. TEC 40 includes TS 30 for sensing the temperature of APD 13 or of cold plate 41. TS 30 provides its temperature readings to TEC controller 44. (g) TEC controller 44. TEC controller 44 is also controlled by a temperature set signal 106 from controller 19.
Device 203 allows at least a limited amount of APD temperature control. TEC controller 44 controls the temperature applied by TEC 40 in order to determine the APD temperature and compensate for changes in the APD temperature.
In voltage domain and for certain applications this configuration has following disadvantages: The DC-DC converter 11 usually has slow load regulation response (in the range of tens KHz). The time response of DC-DC converter's output current limiting circuit is slow (in the range of tens KHz). The DC-DC converter 11 usually has high output ripple and noise.
In temperature domain and for certain applications this configuration has following disadvantages: The time response of such temperature compensation is slow (in the range of hundred Hz), which allow APD application with low gains (in the range of 50) with limited APD currents (about 50 uA). The temperature stabilization performance is limited by finite thermal resistance between APD die and cold plate 41.
FIG. 4 is a cross sectional view of a portion 211 of a prior art device. Portion 211 includes: (a) controller 19, (b) TEC 40, (c) TEC controller 44, (d) intermediate plate 50, (e) APD die 71 that is located within a package that is illustrated as having base 61, housing 63 and window 64. APD die 71 includes APD junction 72 that is light sensitive and faces window 64 and is positioned above electrical insulator 62.
Electrical insulator 62 is electrically insulating but thermally conductive. Electrical insulator 62 is supported by base 61. TEC 40 includes cold plate 41, hot plate 42, solid state devices 43 and TEC controller 44. TEC controller 44 is fed by a control signal from controller 19 and by temperature reading from TS 30 that measures the temperature of the cold plate 41 or of intermediate plate 50. Intermediate plate 50 is connected between cold plate 41 and base 61. Intermediate plate 50 is more massive than cold plate 41 and is used for stabilizing the temperature due to its greater mass. It is noted that if cold plate 41 is big enough then intermediate plate 50 may be omitted.
FIG. 4 also illustrates heat flux 401 that is generated by APD die 71 and propagates through electrical insulator 62, base 61 and intermediate plate 50.
FIG. 5 is a cross sectional view of a portion 212 of a prior art device.
Portion 212 differs from portion 211 by the location of TEC 40 and by using the intermediate plate 50 as a hot plate—instead of being used as a cold plate. TEC 40 is located within the package that surrounds APD die 71. Portion 212 includes controller 19, TEC 40, intermediate plate 50 and APD die 71. APD die 71 is located within a package that includes base 61, housing 63 and window 64. TEC controller 44 may be included inside the package or outside the package.
APD die 71 includes a APD junction 72 that faces window 64. APD die 71 is positioned above electrical insulator 62. Electrical insulator 62 is electrically insulating but is thermally conductive.
TEC 40 is positioned between electrical insulator 62 and base 61 so that cold plate 41 may contact electrical insulator 62 and hot plate 42 may contact base 61. TEC 40 also includes TS 30 and solid state devices 43. Intermediate plate 50 is more massive than hot plate 42. Intermediate plate 50 is used for conducting the heat to an external air or fluid. FIG. 5 also illustrates heat flux 402 that is generated by APD die 71 and propagates through electrical insulator 62 and cold plate 41.
There is a growing need to provide a device that facilitates the APD at high gain values.